Determination of patency of the airway

ABSTRACT

Methods and apparatus for determining the occurrence of an apnea, patency and/or partial obstruction of the airway are disclosed. Respiratory air flow from a patient is measured to give an air flow signal. The determination of an apnea is performed by calculating the variance of the air flow signal over a moving time window and comparing the variance with a threshold value. One determination of partial obstruction of the airway is performed by detecting the inspiratory part of the air flow signal, scaling it to unity duration and area and calculating an index value of the amplitude of the scaled signal over a mid-portion. Alternatively, the index value is a measure of the flatness of the air flow signal over the mid-portion. One determination of patency of the airway is performed by applying an oscillatory pressure waveform of known frequency to a patient&#39;s airway, calculating the magnitude of the component of said air flow signal at the known frequency induced by the oscillatory pressure waveform and comparing the calculated magnitude with a threshold value. Alternatively, the air flow signal is analysed to detect the presence of a component due to cardiogenic activity.

This application is a divisional of application Ser. No. 09/484,761,filed on Jan. 18, 2000, now U.S. Pat. No. 6,675,797 which is acontinuation of application Ser. No. 08/950,322, filed on Oct. 14, 1997,now U.S. Pat. No. 6,029,665, which is a continuation of application Ser.No. 08/335,118 filed Nov. 4, 1994, now U.S. Pat. No. 5,704,345.

FIELD OF THE INVENTION

This invention relates to the detection of the occurrence of an apnea(i.e. the complete cessation of breathing) and to the determination ofairway patency. The condition of patency of the airway is the converseof a total obstruction of the airway. The invention also relates to thedetection of partial obstruction of the airway (i.e. obstructedbreathing). The detection and monitoring of apneas, airway patency andobstruction is advantageous in the diagnosis and treatment ofrespiratory conditions that have adverse effects on a person'swellbeing.

The expression “airway” as used herein is to be understood as theanatomical portion of the respiratory system between the nares and thebronchii, including the trachea. The expression “respiration” is to beunderstood as the continually repeating events of inspiration (inhaling)followed by expiration (exhaling).

BACKGROUND OF THE INVENTION

In the Sleep Apnea syndrome a person stops breathing during sleep.Cessation of airflow for more than 10 seconds is called an “apnea”.Apneas lead to decreased blood oxygenation and thus to disruption ofsleep. Apneas are traditionally (but confusingly) categorized as eithercentral, where there is no respiratory effort, or obstructive, wherethere is respiratory effort. With some central apneas, the airway ispatent, and the subject is merely not attempting to breathe. Conversely,with other central apneas and all obstructive apneas, the airway is notpatent (i.e. occluded). The occlusion is usually at the level of thetongue or soft palate.

The airway may also be partially obstructed (i.e. narrowed or partiallypatent). This also leads to decreased ventilation (hypopnea), decreasedblood oxygenation and disturbed sleep.

The dangers of obstructed breathing during sleep are well known inrelation to the Obstructive Sleep Apnea (OSA) syndrome. Apnea, hypopneaand heavy snoring are recognised as causes of sleep disruption and riskfactors in certain types of heart disease. More recently it has beenfound that increased upper airway resistance (Upper Airway Resistancesyndrome) during sleep without snoring or sleep apnea also can causesleep fragmentation and daytime sleepiness. It is possible there is anevolution from upper airway resistance syndrome to sleep apnea,accompanied by a worsening of clinical symptoms and damage to thecardiovascular system.

The common form of treatment of these syndromes is the administering ofContinuous Positive Airway Pressure (CPAP). The procedure foradministering CPAP treatment has been well documented in both thetechnical and patent literature. Briefly stated, CPAP treatment acts asa pneumatic splint of the airway by the provision of a positivepressure, usually in the range 4-20 cm H₂O. The air is supplied to theairway by a motor driven blower whose outlet passes via an air deliveryhose to a nose (or nose and/or mouth) mask sealingly engaged to apatient's face. An exhaust port is provided in the delivery tubeproximate to the mask. More sophisticated forms of CPAP, such asbi-level CPAP and autosetting CPAP, are described in U.S. Pat. Nos.5,148,802 and 5,245,995 respectively.

Various techniques are known for sensing and detecting abnormalbreathing patterns indicative of obstructed breathing. U.S. Pat. No.5,245,995, for example, describes how snoring and abnormal breathingpatterns can be detected by inspiration and expiration pressuremeasurements while sleeping, thereby leading to early indication ofpreobstructive episodes or other forms of breathing disorder.Particularly, patterns of respiratory parameters are monitored, and CPAPpressure is raised on the detection of pre-defined patterns to provideincreased airway pressure to, ideally, subvert the occurrence of theobstructive episodes and the other forms of breathing disorder.

As noted above, central apneas need not involve an obstruction of theairway, and often occur during very light sleep and also in patientswith various cardiac, cerebrovascular and endocrine conditions unrelatedto the state of the upper airway. In those cases where the apnea isoccurring without obstruction of the airway, there is little benefit intreating the condition by techniques such as CPAP. Also, known automatedCPAP systems cannot distinguish central apneas with an open airway fromapneas with a closed airway, and may inappropriately seek to increasethe CPAP splinting air pressure unnecessarily. Such unnecessaryincreases in pressure reflexly inhibit breathing, further aggravatingthe breathing disorder.

Other limitations associated with the prior art include the inability todetect airway patency and the absence of progressive, heirarchicresponse to increasingly severe indicators of airway obstruction forwhich the mask pressure should be increased.

It would be useful, however, to even more sensitively and reliablydetect the conditions of partial obstruction, as well as apnea andpatency, as this would assist in the design of equipment to preventthese conditions from occurring. In a similar way, means for detectingand monitoring mildly obstructed breathing would be useful in diagnosingand treating Upper Airway Resistance syndrome and monitoring thattreatment is optimal.

SUMMARY OF THE INVENTION

In accordance with a first aspect the invention discloses a method fordetermining the occurrence of an apnea in a patient, the methodcomprising the steps of:

measuring respiratory air flow from the patient as a function of time;

determining the variance of said measured air flow; and

determining from said variance that an apnea is occurring.

The variance can be a moving average over the time window. Further,there can be a further step of comparing the variance with a thresholdvalue, and if the variance falls below the threshold value then an apneais occurring. The measured air flow can be expressed as an air flowsignal. Advantageously, the respiratory air flow is sampled at equallyspaced points in time to give a sampled air flow signal. Further, it canbe the case that the variance must fall below the threshold value for apredetermined period of time before it is determined that an apnea isoccurring. Advantageously the method can comprise the further step ofeither commencing continuous positive airway pressure (CPAP) treatmentor increasing CPAP treatment pressure to the patient if an apnea isoccurring.

In accordance with a further aspect the invention discloses a method fordetecting partial obstruction of the airway of a patient, the methodcomprising the steps of:

measuring respiratory air flow from the patient;

detecting the inspiratory part of said air flow;

normalising said inspiratory part; and

determining an index value of a mid-portion of said normalisedinspiratory part as a measure of partial obstruction.

Conveniently, the index value is determined from the amplitude of themid-portion of the normalised inspiratory part. The index value can bedetermined as the arithmetic mean value of the amplitude in themid-portion. Alternatively, the index value is determined from theflatness of the mid-portion. Yet further, the index value can bedetermined as the root mean square (RMS) deviation of the normalisedinspiratory part in the mid-portion with respect to unity. The RMSdeviation can be compared against a threshold value to determine thedegree of obstruction. Still further, the step of normalising caninclude scaling the inspiratory part to unity duration and unity area.The determination also can be performed over a plurality of inspiratoryevents. In this way a moving mean value of amplitude or a moving RMSdeviation can be formed. The respiratory air flow also can be sampled atspaced instants in time. Advantageously there is the further step ofeither commencing CPAP treatment or increasing CPAP treatment pressureto the patient if there is partial obstruction of the airway

The invention yet further discloses a method for determining the degreeof obstruction of the airway of a patient receiving continuous positiveairway pressure (CPAP) treatment by apparatus for supplying CPAP to thepatient's airway, the method comprising the steps of:

measuring respiratory air flow from the patient to give an air flowsignal;

filtering said air flow signal to reject components at least due torespiration to give a filtered air flow signal having components due topatient snoring and noise of said CPAP apparatus;

predicting a CPAP apparatus noise component of said filtered air flowsignal; and

subtracting said predicted noise component from said filtered air flowsignal to give a snore component signal as a measure of the degree ofobstruction of the airway.

In a preferred form, the filtering step includes bandpass filtering alsoto reject high frequency noise components.

The invention yet further discloses a method for determining patency ofthe airway of a patient, the method comprising the steps of:

applying an oscillatory pressure waveform of known frequency to thepatient's airway;

measuring respiratory air flow from the patient; and

determining that the airway is patent if there is a component of saidair flow at said known frequency induced by said oscillatory pressurewaveform.

Advantageously the air flow component is determined from the amplitudeof the air flow signal, and there is the further step of comparing themagnitude with a threshold value and if the magnitude is greater thanthe threshold value then the airway is declared patent. Furthermore, themethod can be performed when the patient is having an apnea and there iszero air flow. The step of determining can be said to identifymodulation of the measured air flow by the oscillatory pressurewaveform.

The invention yet further discloses a method for determining the degreeof patency of the airway of a patient, the method comprising the stepsof:

applying an oscillatory pressure waveform of known frequency andmagnitude at an entrance to the patient's airway;

measuring respiratory air flow from the patient;

determining the magnitude of the component of said air flow at saidknown frequency induced by said oscillatory pressure waveform; and

determining the degree of patency as the ratio of said induced air flowmagnitude and said oscillatory pressure waveform magnitude.

The measured air flow can be expressed as an air flow signal.Furthermore, the method can be performed when the patient is asleep, andfurther advantageously, when it previously has been determined that thepatient is having an apnea. In the case of an apnea there is zero airflow.

The invention yet further discloses a method for determining patency ofthe airway of a patient, the method comprising the steps of:

measuring respiratory air flow from the patient;

analysing said air flow to detect the presence of cardiogenic air flow,and if said cardiogenic air flow is present then the airway is declaredpatent.

Again, the measured air flow can be expressed as an air flow signal. Therespiratory air flow can be high pass filtered to reject components dueto respiration. Further, the step of analysing detects the presence of aperiodic component. The periodic component can include a fundamentalcomponent together with a sub-multiple or harmonic thereof. The step ofanalysing can further include performing a Fourier transformation on theair flow signal. Conveniently there can be the further step of detectingthe patient's cardiac rate, and whereby the analysing step includesdetecting a component of the air flow at the cardiac rate. Furthermore,the method can be performed when the patient is having an apnea andthese is zero air flow.

The invention yet further discloses a method or controlling theadministration of CPAP treatment to the airway of a patient by meanscontrollable to supply breathable air to the patient's airwaycontinually at a selectable pressure elevated above atmosphericpressure, the method comprising the step of:

-   -   commencing or increasing CPAP treatment pressure if:    -   (a) an apnea is occurring, determined by the steps of:    -   measuring respiratory air flow from the patient as a function of        time; and    -   determining the variance of said measured air flow as an        indication of an apnea occurring;    -   or (b) there is partial obstruction of the airway, determined by        the steps of:    -   measuring respiratory air flow from the patient:    -   detecting the inspiratory part of said air flow:    -   normalising said inspiratory part; and    -   determining an index value of a mid-portion of said normalised        inspiratory part as a measure of partial obstruction;    -   or (c) there is patency of the airway, determined by the steps        of:        -   (i) applying an oscillatory pressure waveform of known            frequency to the patient's airway;        -   measuring respiratory air flow from the patient; and        -   determining that the airway is patent if here is a component            of said air flow at said known frequency induced by said            oscillatory pressure waveform;        -   or (ii) measuring respiratory air flow from the patient; and        -   analysing said measured air flow to detect the presence of            cardiogenic air flow, and if so then the airway is declared            patent.

The invention yet further discloses apparatus for determining theoccurrence of an apnea in a patient, the apparatus comprising:

means for measuring respiratory air flow from the patient as a functionof time;

means for determining the variance of said measured air flow; and

means for determining from said variance that an apnea is occurring.

The invention yet further discloses apparatus for detecting partialobstruction of the airway of a patient, the apparatus comprising:

means for measuring respiratory air flow from the patient;

means for detecting the inspiratory part of said air flow;

means for normalising said inspiratory part; and

means for determining an index value of a mid-portion of said normalisedinspiratory part as a measure of partial obstruction.

The invention yet further discloses apparatus for determining the degreeof obstruction of the airway of a patient receiving continuous positiveairway pressure (CPAP) treatment by means for supplying CPAP to thepatient's airway, the apparatus comprising:

means for measuring respiratory air flow from the patient to give an airflow signal;

means for filtering said air flow signal to reject components at leastdue to respiration to give a filtered air flow signal having componentsdue to patient snoring and noise of said CPAP apparatus;

means for predicting a CPAP apparatus noise component of said filteredair flow signal; and

means for subtracting said predicted noise component from said filteredair flow signal to give a snore component signal as a measure of thedegree of obstruction of the airway.

The invention yet further discloses apparatus for determining patency ofthe airway of a patient, the apparatus comprising:

means for applying an oscillatory pressure waveform of known frequencyto the patient's airway;

means for measuring respiratory air flow from the patient; and

means for determining that the airway is patent if there is a componentof said air flow at said known frequency induced by said oscillatorypressure waveform.

The invention yet further discloses apparatus for determining the degreeof patency of the airway of a patient, the apparatus comprising:

means for applying an oscillatory pressure waveform of known frequencyand magnitude to the patient's airway;

means for measuring respiratory air flow from the patient;

means for determining the magnitude of the component of said air flow atsaid known frequency induced by said oscillatory pressure waveform; and

means for determining the degree of patency as the ratio of said inducedair flow magnitude and said oscillatory pressure waveform magnitude.

The invention yet further discloses apparatus for determining patency ofthe airway of a patient, the apparatus comprising:

means for measuring respiratory air flow from the patient; and

means for analysing said measured air flow to detect the presence ofcardiogenic air flow, and if said cardiogenic air flow is present thenthe airway is declared patent.

The invention vet further discloses apparatus for controlling theadministration of CPAP treatment to the airway of a patient comprising

means controllable to supply breathable air to the patient's airwaycontinually at a selectable pressure elevated above atmosphericpressure;

controlling means for commencing or increasing CPAP treatment pressureif:

-   -   (a) an apnea is occurring, determined by:    -   measuring respiratory air flow from the patient as a function of        time; and    -   determining the variance of said measured air flow as an        indication of an apnea occurring;    -   or (b) there is partial obstruction of the airway determined by:    -   measuring respiratory air flow from the patient;    -   detecting the inspiratory part of said air flow;    -   normalising said inspiratory part; and    -   determining an index value of a mid-portion of said normalised        inspiratory part as a measure of partial obstruction;    -   or (c) there is patency of the airway, determined by:        -   (i) applying an oscillatory pressure waveform of known            frequency to the patient's airway;        -   measuring respiratory air flow from the patient; and        -   determining that the airway is patent if there is a            component of said air flow at said known frequency induced            by said oscillatory pressure waveform;        -   or (ii) measuring respiratory air flow from the patient; and        -   analysing said measured air flow to detect the presence of            cardiogenic air flow, and if so then the airway is declared            patent.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 shows a flow diagram of the basic methodology of an embodiment;

FIG. 2 shows, in diagrammatic form, apparatus embodying the invention;

FIG. 3 shows an alternative arrangement of the apparatus of FIG. 2;

FIG. 4 shows a graph of air flow with time for normal and partiallyobstructed inspiration;

FIG. 5 shows a flow diagram of the determination of an apnea;

FIGS. 6 a and 6 b show a flow diagram of the calculation of the shapefactors;

FIG. 7 shows a flow diagram of an embodiment utilising both shape factormethodologies;

FIGS. 8 a and 8 b show clinical data of CPAP treatment utilising theshape factor methodologies;

FIGS. 9 a-9 c and 10 a-10 c show clinical respiratory air flow andfrequency signals during an apnea;

FIG. 11 shows a flow diagram for the cardiogenic determination ofpatency;

FIGS. 12 a-12 d and 13 a-13 d show graphs of clinical respiratory datademonstrating the detection of patency;

FIG. 14 shows a flow diagram of an applied modulated output in thedetermination of patency;

FIG. 15 shows a flow diagram of leak compensated patency determination;and

FIG. 16 shows, in schematic form, a preferred CPAP treatment system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS AND BEST MODE

FIG. 1 is a flow diagram of the basic methodology of one embodiment. Thefirst step 10 is the measurement of respiratory flow (rate) over time.This information is processed in step 12 to generate Index values to beused as qualitative measures for subsequent processing. Step 14 detectswhether an apnea is occurring by comparison of the Breathing Index witha Threshold value.

If the answer in step 14 is “Yes”, an apnea is in progress and therethen follows a determination of patency in step 20. If there is patencyof the airway, a central apnea with an open airway is occurring, and, ifdesired, the event is logged in step 22. If the result of step 20 isthat the airway is not patent, then a total obstructive apnea or acentral apnea with closed airway is occurring, which results in thecommencement or increase in CPAP treatment pressure in step 18. Ifdesired, step 18 may include the optional logging of the detectedabnormality.

If the answer in step 14 is “No”, the Obstruction Index is compared withanother Threshold value in step 16, by which the determination ofobstruction of the airway is obtained. If “Yes” in step 16, then thereis a partial obstruction, and if “No”, there is no obstruction(normalcy).

Thus step 18 applies in the case of a complete or partial obstruction ofthe airway with a consequential increase in CPAP treatment pressure. Inthe instance of a central apnea with patent airway (steps 20,22) ornormal breathing with no obstruction, the CPAP treatment pressure ratheris reduced, in accordance with usual methodologies that seek to set theminimal pressure required to obviate, or at least reduce, the occurrenceof apneas. The amount of reduction in step 17 may, if desired, be zero.

The methodology represented in FIG. 1 is of a clinical embodiment, wherepatient CPAP pressure is controlled over time as appropriate. A purelydiagnostic embodiment operates in the same manner except it omits theCPAP pressure increase and pressure decrease actions of step 18 and step17 respectively.

FIG. 2 shows, in diagrammatic form, clinical CPAP apparatus inaccordance with one embodiment for implementing the methodology ofFIG. 1. A mask 30, whether either a nose mask and/or a face mask, issealingly fitted to a patient's face. Fresh air, or oxygen enriched air,enters the mask 30 by flexible tubing 32 which, in turn, is connectedwith a motor driven turbine 34 to which there is provided an air inlet36. The motor 38 for the turbine is controlled by a motor-servo unit 40to either increase or decrease the pressure of air supplied to the mask30 as CPAP treatment. The mask 30 also includes an exhaust port 42 thatis close to the junction of the tubing 34 with the mask 30.

Interposed between the mask 30 and the exhaust 42 is a flow-resistiveelement 44. This can take the form of an iris across which adifferential pressure exits. The mask side of the flow-resistive element44 is connected by a small bore tube 46 to a mask pressure transducer 48and to an input of a differential pressure transducer 50. Pressure atthe other side of the flow-resistive element 44 is conveyed to the otherinput of the differential pressure transducer 50 by another small boretube 52.

The mask pressure transducer 48 generates an electrical signal inproportion to the mask pressure, which is amplified by amplifier 52 andpassed both to a multiplexer/ADC unit 54 and to the motor-servo unit 40.The function of the signal provided to the motor-servo unit 40 is as aform of feedback to ensure that the actual mask static pressure iscontrolled to be closely approximate to the set point pressure.

The differential pressure sensed across the flow-resistive element 44 isoutput as an electrical signal from the differential pressure transducer50, and amplified by another amplifier 56. The output signal from theamplifier 56 therefore represents a measure of the mask or respiratoryairflow rate. A large dynamic range can be achieved by using aflexible-vaned iris as the flow-resistive element 44.

The output signal from the amplifier 56 is low-pass filtered by thelow-pass filter 58, typically with an upper limit of 10 Hz. Theamplifier 56 output signal is also bandpassed by the bandpass filter 60,and typically in a range of 30-300 Hz. The outputs from both thelow-pass filter 58 and the bandpass filter 60 are provided to themultiplexer/ADC unit 54. The multiplexed and digitized output from themultiplexer/ADC unit 54 is, in turn, passed to a controller 62,typically constituted by a micro-processor based device also providedwith program memory and data processing storage memory. A component ofthe multiplexed output is a digitized and manipulated form of the airflow signal f(t), represented as f_(n).

Dependant upon the specific processing functions it performs, thecontroller 62 outputs a pressure request signal which is converted by aDAC 64, and passed to the motor-servo unit 40. This signal thereforerepresents the set point pressure (P_(set)(t)) to be supplied by theturbine 34 to the mask 30 in the administration of CPAP treatment.

The controller 62 is programmed to perform a number of processingfunctions, as presently will be described.

As an alternative to the mask pressure transducer 48, a directpressure/electrical solid state transducer (not shown) can be mountedfrom the mask with access to the space therewithin, or to the airdelivery tubing 32 proximate the point of entry to the mask 30.

Further, it may not be convenient to mount the flow transducer 44 at ornear the mask 30, nor to measure the mask pressure at or near the mask.An alternative arrangement, where the flow and pressure transducers aremounted at or near the air pressure generator (in the embodiment beingthe turbine 34) is shown in FIG. 3.

The pressure p_(g)(t) occurring at the pressure generator 34 outlet ismeasured by a pressure transducer 70. The flow f_(g)(t) through tubing32 is measured with flow sensor 72 provided at the output of the turbine34.

The pressure loss along tubing 32 is calculated in step 74 from the flowthrough the tube f_(g)(t), and a knowledge of the pressure-flowcharacteristic of the tubing, for example by table lookup.

The pressure at the mask P_(m) is then calculated in subtraction step 76by subtracting the tube pressure loss from P_(g)(t).

The pressure loss along tube 32 is then added to the desired setpressure at the mask p_(set)(t) in summation step 78 to yield thedesired instantaneous pressure at the pressure generator 34. Preferably,controller of the pressure generator 34 has a negative feedback inputfrom the pressure transducer 70, so that the desired pressure from step78 is achieved more accurately.

The flow through the exhaust 42 is calculated from the pressure at themask (calculated in step 76) from the pressure-flow characteristic ofthe exhaust step 80, for example by table lookup.

Finally, the mask flow is calculated by subtracting the flow through theexhaust 42 from the flow through the tubing 32, in subtraction step 82.

The methodology put into place by the controller 62 will now bedescribed with reference to the apparatus of FIG. 2.

A. Determination of Apnea

This section generally corresponds to steps 10, 12, and 14 as shown inFIG. 1.

Partial upper airway obstruction in untreated or partially treatedObstructive Sleep Apnea syndrome, and the related High Airway Resistancesyndrome, leads to mid-inspiratory flow limitation, as shown in FIG. 4,which shows typical inspiratory waveforms respectively for normal andpartially obstructed breaths.

As discussed previously, the respiratory air flow is determined by meansof the differential pressure transducer 48, and a signal representingthe air flow is continuously digitized and passed to the controller 62.If necessary, the controller 62 can linearise the flow signal, forexample, by a table lookup. Occasionally, complete obstruction of theairway can occur unexpectedly for example in a previously untreatedpatient, without a period of preceding partial obstruction.Consequently, the processing steps 12, 14 shown in FIG. 1 also detectthe presence of complete or near-complete cessation of air flow, orapnea, using the measure of the Breathing Index in step 14.

This is achieved, for example as shown in FIG. 5, by low-pass filteringof the mask air flow signal f_(n) by low-pass filter element 125,typically with a 1 Hz cutoff, and calculating the moving averagevariance by the computational block 126.

The Breathing Index at any given point in time is calculated as thesquare root of the variance of the digitized flow signal, f_(n):

${{breathing}\mspace{14mu}{index}} = \sqrt{\frac{{\sum\limits_{i = 0}^{I - 1}\; f_{n - i}^{2}} - {\frac{1}{I}\left( {\sum\limits_{i = 0}^{I - 1}\; f_{n - i}} \right)^{2}}}{I}}$where  I = 2 ⋅ sample  rate            

The average variance calculated over a moving time window is comparedwith a Threshold by the level detector 127, to generate an“airflow-ceased” trigger. This starts the timer 128. If the triggerpersists for more than 10 seconds, the comparator 129 declares an apnea.The Threshold may be a fixed value, typically 0.1 l/sec, or may be achosen percentage (typically 10 or 20%) of the average ventilation overthe last several minutes (typically 5 minutes). For convenience, insteadof comparing the Threshold with the square root of the variance, one cansquare the Threshold, and compare with the variance directly.

Conversely, if airflow resumes before 10 seconds lapses, the timer 128is reset and no apnea is declared. If an apnea is taking place, thepatency of the airway must also be determined as an indicator of whetherthe apnea is of the central type with open airway, or otherwise. Theprocessing performed by the controller 62 to achieve this determinationwill be discussed presently.

The method can, of course, be used instantaneously without requiring theelapse of a time interval before an apnea is declared.

The method is advantageous in comparison with known methods fordetecting apnea, such as single zero crossing methods, because it isrelatively insensitive to leaks. Furthermore, apneas are still detectedin the presence of cardiogenic, as opposed to respiratory, air flow.

B. Determination of Airway Obstruction

The Obstruction Index is calculated in step 12. Either of two alternateObstruction indices can be calculated. These will be referred to asshape factor 1 and shape factor 2.

The Obstruction Index is then compared with a threshold in step 16 ofFIG. 1. If the obstruction Index is less than the threshold value, CPAPtreatment pressure is increased in step 18. Otherwise, the CPAP pressuremay be reduced in optional step 17.

As shown in FIG. 6 a, the digitized airflow signal, f_(n), has anycomponents below 0.1 Hz due to leaks of the mask 30 subtracted by ahigh-pass filter 90. The inspiratory and expiratory portions of eachbreath are then identified by a zero-crossing detector 92. A number ofevenly spaced points (typically sixty-five), representing points intime, are interpolated by an interpolator 94 along the inspiratoryflow-time curve for each breath. The curve described by the points isthen scaled by a scaler 96 to have unity length (duration/period) andunity area to remove the effects of changing respiratory rate and depth.

Conveniently, the scaled breaths are compared in a comparator 98 with apre-stored template representing a normal unobstructed breath. Thetemplate is very similar to the curve for a normal inspiratory event asshown in FIG. 4. Breaths deviating by more than a specified threshold(typically 1 scaled unit) at any time during the inspiration from thistemplate, such as those due to coughs, sighs, swallows and hiccups, asdetermined by the test element 100, are rejected.

For data for which the test is satisfied, a moving average of the firstsuch scaled point is calculated by the arithmetic processor 102 for thepreceding several inspiratory events. This is repeated over the sameinspiratory events for the second such point, and so on. Thus, sixtyfive scaled data points are generated by the arithmetic processor 102,and represent a moving average of the preceding several inspiratoryevents. The moving average of continuously updated values of the sixtyfive points are hereinafter called the “scaled flow”, designated asf_(s)(t). Equally, a single inspiratory event can be utilised ratherthan a moving average.

From the scaled flow two shape factors that directly relate to thedetermination of partial obstruction are calculated. Each shape factorequates to the Obstruction Index discussed above.

Shape factor 1 is the ratio of the mean of the middle thirty-two scaledflow points to the mean overall sixty-five scaled flow points. This isthus a determination of the reduction of the magnitude (depression) ofthe mid-portion of the scaled inspiratory event(s). Since the mean forall sixty five points is unity, the division need not actually beperformed.

Mathematically, it is expressed as:

${{shape}\mspace{14mu}{factor}\mspace{14mu} 1} = {\sum\limits_{t = 16}^{48}\;{{f_{s}(t)}/{\sum\limits_{t = 1}^{65}\;{f_{(s)}(t)}}}}$

which reduces simply to

$\sum\limits_{t = 16}^{48}\;{{f_{s}(t)}.}$

For a normal inspiratory event this ratio will have an average value inexcess of unity, because a normal such inspiratory event is of higherflow in the middle than elsewhere, as can be seen from FIG. 4.Conversely, for a severely flow-limited breath, the ratio will be unityor less, because flow limitation occurs particularly during the middlehalf of the breath when the upper airway suction collapsing pressure ismaximal. A ratio of 1.17 is taken as the Threshold value (step 16 ofFIG. 1) between partially obstructed and unobstructed breathing, andequates to a decree of obstruction that would permit maintenance ofadequate oxygenation in a typical user.

In other embodiments the number of sampled points, number of breaths andnumber of middle points can be varied, and still achieve a meaningfuldetermination of whether partial obstruction is occurring. The Thresholdvalue similarly can be a value other than 1.17.

Alternatively, the second shape factor is calculated as the RMSdeviation from unit scaled flow, taken over the middle thirty twopoints. This is essentially a measure of the flatness of the mid-portionof the scaled respiratory event(s). Expressed mathematically, this is:

${{shape}\mspace{14mu}{factor}\mspace{14mu} 2} = {\sqrt{\frac{\sum\limits_{t = 16}^{48}\left( {{f_{s}(t)} - 1} \right)^{2}}{32}}.}$

For a totally flow-limited breath, the flow amplitude vs. time curvewould be a square wave and the RMS deviation would be zero. For a normalbreath, the RMS deviation is approximately 0.2 units, and this deviationdecreases as the flow limitation becomes more severe. A threshold valueof 0.15 units is used in step 16 of FIG. 1.

Both shape factors discussed above can be utilised independently inimplementing the methodology carried by the apparatus of FIG. 2, andresult in the sensitive and reliable detection of partially obstructedbreathing. Better performance again is obtained by implementing bothshape factors executed by the controller 62 so that both shapeparameters act together. In this case, shape factor 2 is preferred foruse to detect all but the most severe obstructions, and shape factor 1therefore is preferred for detecting only the most severe obstructions,achieved by reducing the critical threshold from 1.17 to 1.0.

FIG. 7 is a flow diagram illustrating the principle of the two shapefactors operating in concert. The scaled flow signal f_(s)(t) isprovided to a shape detector 112, such as has been described withreference to FIGS. 6 a and 6 b. The shape detector 112 generates shapefactor 1 and shape factor 2. Shape factor 1 is applied to a decisionblock 114 and compared against the Threshold value of 1.0. If theoutcome of the comparison is “Yes”, then it is determined that thereshould be an increase in the CPAP pressure setting, as indicated inblock 116. The shape factor 2 is provided to the decision block 118, anda comparison made against the Threshold value of 0.15. If the answer is“Yes”, then it also is appropriate for an increase in the CPAP pressure,as shown in block 120.

In either case, if the results of the comparison is “No”, then thoseresults are ANDed in the AND gate 122. That is, an output will only beachieved if both Threshold criteria are not satisfied. In this case,there is no partial obstruction, or partial obstruction has subsided, inwhich case, as indicated in block 124, it is appropriate to decrease theCPAP pressure.

This arrangement avoids any peculiarities affecting either algorithm.For example, the presence of an initial non-flow-limited period early ina breath can permit an early sharp peak in the flow-time curve. Thismeans that the scaled flow during the middle half of the breath may bebelow unity. For very severely obstructive breaths, the RMS deviationfrom unity may therefore rise again, and shape factor 2 will fail torecognise such breaths. They will, however, be correctly identified bythe now desensitized shape factor 1. Some normal breaths can involve aninspiratory flow-time waveform approximating a right triangle, where themean flow during the middle half of the inspiration is close to unity.Such a waveform correctly triggers neither shape factor 1 nor shapefactor 2. That is, the instantaneous flow during the middle half of theinspiration is only unity at a single point, and above or below unityelsewhere, so the RMS deviation from unit scaled flow will be large.

In summary, the shape factors provide an Index of the state of theairway. They provide a sensitive warning of an airway becoming unstable,and allow early CPAP treatment to occur. Continuing calculation of themoving average shape, and thus the shape factors, provides an accurateon-going assessment of the degree of any such apnea that is notsubverted by CPAP treatment in order that modified appropriate treatmentor corrective action can be taken.

The shape factors discussed above provide the most sensitive indicationof upper airway stability and therefore result in the smallest increasein the CPAP pressure that should restore stability to the airway andsimilarly a correspondingly small decrease in the CPAP pressure whenstability has so been restored. By being able to maintain the increasesto such a small level, the patient is less likely to be woken, and willalso benefit from avoiding apneas with their associated health risks.

For example, when shape factor 1 is below 1.0, the CPAP pressure isincreased in proportion to the amount of the ratio being below 1.0. Anincrease of 1 cm H₂O per breath per unit below a ratio of 1.0 has beenfound particularly effective. Conversely, if the ratio is above 1.0, theCPAP pressure is gradually reduced with a time constant of 20 minutes.If shape factor 2 is below 0.2, the CPAP pressure is increased at a rateof 1 cm H₂O per breath per unit below 0.2. Conversely, if the shapefactor is above 0.2 units, the pressure is gradually lowered with a timeconstant of 20 minutes.

An example of experimental validation involved a subject with severeObstructive Sleep Apnea syndrome placed on nasal CPAP therapy. Acatheter tip pressure transducer was placed in the hypopharyngeal space,below the site of upper airway obstruction, and the peak upper airwaypressure gradient (UAP) from hypopharynx to mask calculated for eachbreath.

The CPAP pressure was intentionally reduced from time to time duringstable sleep, in order to produce partial upper airway obstruction. Foreach breath taken during the night, the two shape factors werecalculated, and plotted against the UAP, measured in cm H₂O. The resultsare shown in FIGS. 8 a and 8 b.

In this patient there was an 83% correlation between shape factor 1(FIG. 8 a) and UAP, with low values of shape parameter one associatedwith a high pressure drop across the upper airway, indicating partialobstruction. Similarly, there was an 89% correlation between shapefactor 2 (FIG. 8 b) and UAP.

The function achieved by shape factor 1 also can be achieved by animproved methodology in the detection of snoring.

Prior art U.S. Pat. No. 5,245,995 describes signal processing of themask flow signal to determine a snore characteristic, particularly asshown in FIGS. 9 and 10 of that document. The respiratory air flowsignal is bandpass filtered in the range 30-300 Hz. Snoring exhibitscharacteristic frequencies in this range, and as described in the priorart reference the sound intensity of snoring is indicative of almostcomplete obstruction of the airway. Thus CPAP pressure is increased ifthe snore signal is in excess of a snore threshold value. This thencorresponds to the degree of obstruction otherwise detected by shapefactor 1.

Although the snore detector and CPAP treatment effected in consequenceof the occurrence of snoring operates satisfactorily there is stillscope for improvement. Once particular problem comes in that some CPAPapparatus caused wind noise occurs in the 30-300 Hz range, as doesbackground noise due to the motor driving the blower.

As described herein, the digitized flow signal f_(n) has been arrived atin a similar manner to that described in prior art U.S. Pat. No.5,245,995, and thus includes snore component frequencies.

The methodology to improve performance of the snore detector firstlyinvolves a determination of the blower motor speed. This can be achievedby a tachometer located on the motor. Then follows a determination of anexpected flow signal such as would occur in the absence of snoring. Thisis calculated as a function of motor speed and airflow by the followingformula:

${{predicted}\mspace{14mu}{signal}} = {{k_{1}\omega} + {k_{2}\omega^{2}} + {k_{3}f} + {k_{4}{\frac{\mathbb{d}f}{\mathbb{d}t}.}}}$where ω is the motor speed signal and f is the flow signal. Theconstants k₁-k₄ are determined empirically. The predicated signal isthen subtracted from the measured flow signal to obtain the snoresignal. Thus the corrected snore signal more accurately reflects theoccurrence and extent of snoring, and when compared against the snorethreshold results in an increase in the CPAP pressure.C. Determination of Airway Patency

If the outcome of step 14 is “Yes”, then an apnea in progress. Inaccordance with the methodology of FIG. 1, a determination of airwaypatency (step 20) is made. Two methods are now described. The first is ameasurement by cardiogenic airflow, and the second is an externallyinduced oscillation technique.

1. Cardiogenic Airflow

With each beat of the heart, of the order of 66 ml of blood is ejectedfrom the chest over about 0.3 sec, producing a pulsatile blood flow outof the chest of the order of 0.22 l/sec peak flow. If the chest wallwere rigid this would create a partial vacuum in the chest cavity, and,if the upper airway were open and of zero resistance, a similar quantityof air would be sucked in through the trachea.

In practice, the chest wall is not totally rigid, and the upper airwayhas a finite resistance. Consequently the observed airflow with eachbeat of the heart is of the order of 0.02 to 0.1 l/sec. If there is acentral apnea with an open airway, there will be a very small pulsatileairflow of the order of 0.02 to 0 l/sec in time with the heart beat.Conversely, if the airway is closed, there will be no pulsatile airflowin time with the heart beat.

FIGS. 9 a-9 c represent a central apnea with an open airway lastingapproximately 30 seconds, determined from diaphragm electromyogramtracings (not shown). Conversely, FIGS. 10 a-10 c represent anobstructive apnea with a closed airway. FIGS. 9 a and 10 a respectivelyshow a respiratory airflow signal, f(t), during which an apnea lastingapproximately 25 seconds occurs, indicated by a near cessation ofairflow.

FIGS. 9 b and 10 b respectively show a ten second close-up (betweent=11.5 s to t=21.5 s) of the airflow signal during the apnea. It can benoted that in FIG. 9 b, where the airway is open, small rhythmicoscillations in the airflow are seen, with the expected peak flow ofabout 0.11/sec. Inspection of the corresponding electrocardiogram (notshown) confirms that these oscillations are of cardiac origin, withairflow either phase-locked with the heartbeat, or at exactly double thecardiac rate. Conversely, in FIG. 10 b, there is either no airflow atall, or at least irregular airflow due to not quite completeobstruction.

FIGS. 9 c and 10 c respectively show the discrete Fourier transform ofFIGS. 9 b and 10 b. In FIG. 9 c (open airway), there are strong peaks inthe frequency spectrum at around 1.25 Hz and/or 2.5 Hz corresponding tothe heart rate and its first harmonic. The peaks reach an amplitude ofat least 0.01 L/sec. Conversely, in FIG. 10 c (closed airway), thediscrete Fourier transform shows little or no activity between 0.75 and3 Hz.

The methodology firstly records the airflow, f(t), using by the flowtransducer 48 shown in FIG. 2 or FIG. 3. The signal is digitized, forexample at 50 Hz, using the analog-to-digital converter (ADC) 54, andsampled by the controller 62. The subsequent processing steps are shownin FIG. 11.

If required, the flow signal, f_(n), is digitally bandpass filtered bythe bandpass filter 130 between 0.1 and 6 Hz to remove low frequencycomponents (leak) and high frequency components (noise) to yield a cleanrespiratory air flow signal.

The occurrence of an apnea will have previously been determined by, forexample, the Breathing Index derived in FIG. 5. In that case the processcontinues.

A Discrete Fourier-transform (DFT) is performed, by the processingelement 132, of the airflow signal fn during the apnea. Only terms up to6 Hz need to be calculated. In the case where the heart rate is notknown, processing is as follows: if the amplitude of the DFT exceeds aThreshold value of 0.01 L/sec, as determined by the peak height detector136 and the subsequent comparator element 138, at any frequency between0.75 and 3 Hz (bandpass element 134), the airway is declared open;otherwise it is declared closed. Patency Index 1 represents the outputof the peak height detector 136.

If an electrocardiogram or other indicator of heartbeat, such as a pulseoximeter is available, then an appropriate method is to:

(1) Use a digital or electronic trigger to trigger on each heart beat.

(2) Accumulate the respiratory airflow signal at time nT after receiptof each trigger into element n of an array, summing with previous valuesat time nT for the duration of the apnea.

(3) Divide by the number of heartbeats to obtain the average air flow asa function of time into the heartbeat.

(4) Calculate the first two terms of the DFT of this signal (fundamentaland first harmonic) and inspect for an amplitude of the order of 0.1l/sec.

In such a case where the heart rate is known, then only the amplitudesat the heart rate and its first harmonic need be considered, leading toa more accurate estimation.

Instead of using the DFT, any suitable mathematical method of detectinga rhythmic oscillation with a frequency of the anticipated heart rateand its first harmonic (0.75 to 3 Hz) will suffice. Such methods couldinclude measuring the regularity of peak heights and zero crossings,autocorrelation, or other digital filtering methods.

2. Externally Induced Oscillations

If the airway is open, but the respiratory muscles are relaxed (i.e. acentral apnea with open airway), then small externally originatingfluctuations in the mask pressure will induce a small respiratoryairflow by inflating and deflating the lungs, and by compressing anddecompressing the gas in the lungs. Conversely, if the airway is closed,no airflow will be induced.

FIG. 12 a shows a respiratory airflow signal as a function of timeduring nasal CPAP therapy. In the first half of the tracing, there is acentral apnea with open airway lasting approximately 22 seconds. FIG. 12b shows that the CPAP pressure is approximately 15.5 cm H₂O. The highfrequency “noise” apparent through most of the pressure trace is largelydue to cardiogenic airflow as previously discussed.

Approximately 5-seconds into the apnea a 2 Hz, 1 cm H₂O pressureoscillation is induced (applied) for 6 seconds (i.e. between t=14 s tot=20.5 s). It can be seen that this pressure modulation induces acorresponding 2 Hz modulation in the respiratory air flow signal. FIGS.12 c-12 d are an enlargement of the period of testing. The respiratoryair flow signal has an amplitude of approximately +0.2 l/sec.

Conversely, in FIGS. 13 a-13 d there is an obstructive apnea, with aclosed airway. A similar tracing would be seen with a central apnea witha closed airway. It can be seen that in this case there is no obviousinduced flow signal during the 6 second period of 2 Hz pressureoscillations. The mean induced signal was 0.01 l/sec.

The procedure is typically, at 4-6 seconds into the apnea, the CPAPpressure generator output pressure supplied to the motor-servo unit 40is controlled to produce a modulated pressure output. As shown in FIG.14, the output from the generation element 140 (controller 62) is asignal modulated with a low amplitude square wave, typically at 2-4 Hz.This produces a quasi-sinusoidal oscillation in the mask pressure with atypical amplitude of 0.5-1 cm H₂O.

As further shown in FIG. 14, the air flow induced by the pressuremodulation is separated from air flow induced by other factors (such asheartbeat), by demodulating the measured air flow signal, f_(n), by ademodulator 142 with the 2 Hz driving signal. The components at 0degrees and 90 degrees to the output signal are calculated, and theiramplitudes are added vectorially to yield a mean induced air flow signalamplitude (Patency Index 2). The mean signal in this case is 0.12 l/sec.

Apneas are classified as “airway open” if the mean induced signal ismore then 0.03 l/sec, and “airway closed” if the mean induced signal isless than 0.03 l/sec. Alternatively, the mean induced signal could bedivided by the amplitude of the inducing pressure to yield theconductance (degree of openness) as a continuous variable.

When it is desired to determine the state of the airway in the presenceof typical CPAP treatment, it is preferable to take into account theeffect of mask leaks. A leak between the mask and the face can produce afalse positive induced air flow signal. As shown in FIG. 15, theoscillator 140 induces the low-frequency, low amplitude pressureoscillations as previously described. The air flow signal f_(n) is highpass filtered by the high pass filter 148 (typically 0.1 Hz) to removeleak and passed to the demodulator 146, which produces Patency Index 2as previously described.

The flow signal is also low pass filtered (typically 0.1 Hz) by the lowpass filter 150 to derive a measurement of leak. The value calculated instep 142 represents the sum of the induced signal due to modulation ofrespiratory air flow and the induced signal due to modulation of flowthrough the leak. The induced signal due to modulation of flow throughthe leak is then calculated by arithmetic element 154, as:

$\frac{{0.5 \cdot {leak} \cdot {inducing}}\mspace{14mu}{oscillation}\mspace{14mu}{amplitude}}{{mean}\mspace{14mu}{mask}\mspace{14mu}{pressure}}.$This is then subtracted by the subtractor 156 from the uncompensatedPatency Index to produce a leak-compensated Patency Index. Theleak-compensated Patency Index can optionally be divided by the inducingoscillation amplitude to yield airway conductance, as describedpreviously.

In the case of either methodology utilised to determine patency, if theresult of that determination (step 20) is “No”, then as was the case fora partial obstruction, the CPAP treatment pressure is increased. If theresult is “Yes”, then a central apnea with an open airway is occurring,and it is inappropriate to increase CPAP pressure. Instead the event isonly logged, and step 17 follows, whereby CPAP pressure is reduced, ashas previously been discussed.

3. Extensions to the Methodology of Determining Patency

(1) Instead of declaring the airway open or closed, the airway can bedeclared open to a certain degree. For example, if the peak amplitude ofthe DFT was 50% of the threshold, the airway is taken as being patent todegree 0.5. Similarly with the externally induced oscillation method.

(2) Instead of using the entire duration of the apnea, calculations canbe performed on a moving window of appropriate duration, such as 10seconds. In this way, mixed apneas, in which the airway is open for onlypart of the apnea, can be detected.

(3) Other methods of measuring or inferring respiratory airflow can beutilised. For example, instead of measuring mask airflow with aflow-resistive element and differential pressure transducer, maskairflow could be measured using an ultrasonic flow transducer, orinferred from mask pressure, using a single ended pressure transducer.Alternatively, measurements of chest wall and/or abdominal movement(such as magnetometers, inductance plethysmography, or strain gauges)could be used.

D. A Combined System for Automatic Adjustment of CPAP Pressure

FIG. 16 illustrates, in schematic block form, a particular preferredembodiment of CPAP treatment apparatus. The CPAP machine 164 representsthe component element shown in FIG. 2 or FIG. 3 except for the elementsbearing the reference numerals 54,58,60 and 62. All of the logic blocks166-176 are processing steps implemented in a microcontroller, which, inFIG. 2, is referred to by the reference numeral 62. The embodimentimplements a hierarchic methodology based around the methodology of FIG.1, that allows the progressive use of pre-obstructive and obstructiveindications to trigger CPAP treatment pressure increases of magnitudeand duration appropriate for the severity of the event.

The mask pressure is initially set to a low pressure, typically 4 cmH₂O. Whenever the apnea detector 168 detects an apnea, the airwaypatency detector 170 determines whether the airway is open or closed bythe forced oscillation method, and if closed, the mask pressure isincreased, typically by 1 cm H₂O per 15 seconds of apnea. If a centralapnea is occurring, no increase in CPAP pressure is instructed.

If a snore is detected by the snore detector 172 (such as that disclosedin U.S. Pat. No. 5,245,995) the mask pressure is also increased. If thesnore index on the given breath exceeds a critical threshold value, thepressure is increased by 1 cm H₂O per unit above the threshold value.The defaults threshold for the snore index is 0.2 units, correspondingapproximately to a snore that can only just be reliably detected by atechnician standing at the bedside. The rate of rise in pressure islimited to a maximum of 0.2 cm H₂O per second, or 12 cm H₂O per minute.

In some patients, it is not possible to prevent the occasional snore,even at maximum pressure. Consequently, above pressures of 10 cm H₂O, aheuristic methodology is used to perform a trade-off between thepossible advantage of increasing the pressure and the disadvantage ofincreased side effects. Thus the threshold is adjusted as follows:

Pressure (cm H₂O) Threshold (snore units) Description <10 0.2 very soft10-12 0.25 12-14 0.3 soft 14-16 0.4 16-18 0.6 moderate >18 1.8 loud

If the shape factor 2 is less than the threshold value, the maskpressure also is increased. The default threshold value is 0.15 units.The default rate of increase of pressure is such that a severelyabnormal shape factor of 0.05 units will produce a rise in pressure of0.3 cm H₂O per breath, or approximately 4.5 cm H₂O per minute.

The lips and tongue can sometimes act like a one-way valve, forming aseal during inspiration when the pharyngeal pressure is lowest butfailing during early to mid-expiration when the pressure is highest.Large leaks, and particularly valve-like leaks, can cause the shapefactor to read low, falsely implying flow limitation. To compensate forthis, the default threshold is increased according to an empiracalheuristic technique if there is a large leak, or if there is avalve-like leak. This is to avoid the treatment pressure being increasedunnecessarily. Consequently, in the presence of a large leak, morereliance is placed on the snore and apnea detectors.

In some patients, the shape factor does not become normal even atmaximum pressure. Consequently, a further heuristic trade-off is madebetween possible increases in patency within increasing pressure, versusincreasing side effects.

The heuristics used are as follows:

(i) If the leak exceeds 0.7 l/sec, the critical threshold for the shapefactor is 0. In the range 0.3-0.7 l/sec, the threshold is decreasedproportionately, so that as the leak increases more severe flattening isrequired before the pressure will rise.

(ii) An index of the presence of valve-like leaks is calculated as theratio of the peak flow during the first 0.5 seconds of expiration to themean flow during the second 0.5 seconds of expiration. If this ratioexceeds 5:1, the threshold is 0. In the range 4:1 to 5:1, the thresholdis reduced proportionately.

(iii) If the mask pressure is 20 cm H₂O, the threshold is 0, and isreduced proportionately in the range 10-20 cm H₂O. For example, if theleak is 0.4 l/sec. and the mask pressure is 15 cm H₂O, the threshold isreduced by 25% because of the leak, and a further 50% because of thealready high treatment pressure so that the new threshold is 0.056units. Conversely, if no abnormality is detected on a particular breath(block 176), the mask pressure is reduced with an appropriate timeconstant typically 10-20 minutes per cm H₂O for snore or shape factorchanges, and preferably about 40 minutes per cm H₂O following apneas.

The preferred embodiment of the combined system for automatic adjustmentof CPAP treatment pressure described above was used to treat 28 patientswith previously untreated obstructive sleep apnea syndrome. CPAPpressure commenced at 4 cm H₂O, and increased automatically in responseto closed airway apneas, snoring, and inspiratory air flow limitation.The following table compares results with those obtained in the samesubjects without treatment:

Untreated Treated (mean ± SEM) (mean ± SEM) Apnea Index (events/hr) 35.5± 5.9  1.5 ± 0.32 Time in Apnea (Percent of night) 24.5 ± 4.7  1.0 ±0.37 Slow Wave Sleep (Percent of night)  7.0 ± 1.6 20.0 ± 2.2  REM Sleep(Percent of night)  9.4 ± 1.4 20.3 ± 2.1  Arousal Index (Events/hr) 55.9± 5.3 10.8 ± 1.9  Respiratory Arousals (Events/hr)  5l.5 ± 5.4  4.2 ±1.5

There was a dramatic reduction in the number of apneas per hour, and thepercentage of time in apnea. There was a large increase in thepercentage of deep restorative sleep (slow wave and REM sleep). Therewas a dramatic reduction in the number of arousals from sleep,particularly those of a respiratory origin. These results confirm thatthe combined system produces excellent results in treating obstructivesleep apnea syndrome.

The system described can also be utilised in a diagnostic mode,typically where nasal cannulae are utilized in the place of a maskarrangement sealed to the patient's face. In this mode, measurements ofapneas, patency, and partial obstruction are logged, but no CPAPtreatment is effected. The nasal cannulae are connected to one side ofthe flow sensor 50 in FIG. 2. Only elements 50, 54, 56, 58, 60 and 62are required in this mode. Since with nasal cannulae, the signal fromthe flow transducer 50 is not linear with flow, there is an additionalstep in which the signal from the flow transducer is linearized,preferably by use of a lookup table in the microcontroller 62. The datacollected provides the physician with the ability to diagnose conditionssuch as Obstructive Sleep Apnea syndrome and Upper Airway Resistancesyndrome.

Numerous alterations and modification, as would be apparent to oneskilled in the art, can be made without departing from the basicinventive concept.

More complex variants of CPAP therapy, such as bi-level CPAP therapy ortherapy in which the mask pressure is modulated within a breath, canalso be monitored and/or controlled using the methods described herein.

The moving average variance apnea detector, as described, can beextended to include a hypopnea detector by adding a second comparatorset at a higher threshold, so that it will respond to partial reductionsin ventilation.

1. A method for determining patency of the airway of a patient duringthe delivery of continuous positive airway pressure treatment, themethod comprising the steps of: measuring respiratory air flow from apatient; determining airway patency by an analysis of said measured airflow to detect the presence of card ogenic air flow; and deliveringairway treatment pressure based upon said determination of airwaypatency.
 2. The method of claim 1 wherein said airway treatment pressureis increased if said cardiogenic air flow is not present.
 3. The methodof claim 2 wherein said airway treatment pressure is decreased orunchanged if said cardiogenic air flow is present.
 4. The method ofclaim 3 further comprising the step of filtering said respiratoryairflow to reject unwanted components of respiration.
 5. The method ofclaim 1 wherein said analysis includes performing a fourier transform onsaid measured air flow.
 6. The method of claim 5 further comprising thestep of rate determining the patient's cardiac rate, and said ratedetermining step includes detecting a component of said air flow at thecardiac rate.
 7. An apparatus for determining patency of an airway of apatient, the apparatus comprising: a pressure transducer for generatingan air flow signal representative of respiratory air flow from thepatient; and a processor programmed to determine if the airway is patentby an analysis of said air flow signal to detect the presence ofcardiogenic air flow.
 8. The apparatus of claim 7 further comprising aturbine controllable to provide a supply of breathable gas at a desiredpressure elevated above atmospheric wherein said desired pressure isincreased if said cardiogenic air flow is not present.
 9. The apparatusof claim 8 wherein said desired pressure is decreased or unchanged ifsaid cardiogenic air flow is present.
 10. The apparatus of claim 9wherein said analysis includes performing a Fourier transform on saidmeasured air flow.